1. Field of the Invention
The present invention relates to a heat-generating element and a heating device for heating air, comprising at least one PTC element and electric conductors which are in contact with opposed lateral surfaces of the PTC element. Such a heat-generating element is known e.g. from EP 1 061 776 which is owned by the present applicant.
2. Description of the Related Art
The heat-generating element is used in particular in an auxiliary heating for a motor vehicle and comprises a plurality of PTC elements arranged successively in a row and supplied with current through electric conductors which extend parallel to one another and which are in areal contact with opposed sides of the PTC elements. The conductors are normally formed by parallel sheet metal strips. The thus formed heat-generating elements are used e.g. in a heating device used for heating air in a motor vehicle and comprising several layers of heat-generating elements whose opposed sides are in contact with heat-emitting elements. Making use of a fixture device, these heat-emitting elements are applied to the heat-generating elements in a comparatively good heat-transmitting contact therewith.
In the above-mentioned prior art, the fixture device of the heating device is defined by a frame in which a plurality of parallel layers of heat-generating and heat-emitting elements are held under spring pretension. According to an alternative embodiment, which also discloses a heating device of the type in question and which is described e.g. in EP 1 467 599, the heat-generating element is defined by a plurality of PTC elements arranged successively in a row in one plane; these PTC elements are also referred to as ceramic elements or cold conductors and have current supplied thereto at opposed lateral surfaces through conductors which are in contact with these lateral surfaces. One of these conductors is defined by a circumferentially closed profile. The other conductor is defined by a sheet metal strip which rests on said circumferentially closed metal profile via an interposed electrically insulating layer. The heat-emitting elements are defined by lamellae, which are arranged in a plurality of parallel layers and which extend at right angles to the circumferentially closed metal profile. In the case of the heating device of the type in question known from EP 1 467 599, a plurality of circumferentially closed metal profiles having the structural design described hereinbefore is provided, said circumferentially closed metal profiles being arranged parallel to one another. The lamellae extend partly between the circumferentially closed profiles and they partly project beyond these profiles.
The above-mentioned heat-generating elements necessitate a good electric contact between the electric conductors and the PTC elements. Otherwise, there will be the problem of an increased transfer resistance which, especially when the heat-generating elements are used in auxiliary heatings for motor vehicles, may result in local overheating due to the high currents. This thermal event may cause damage to the heat-generating element. In addition, the PTC elements are self-regulating resistance heaters which emit less heat in response to an increase in temperature, so that local overheating may cause a failure of the self-regulating characteristics of the PTC elements.
Moreover, high temperatures in the area of an auxiliary heating may lead to a development of fumes or gases that may be directly hazardous to the health of persons in the passenger compartment.
Just as problematic is the use of the heat-generating elements of the type in question at high operating voltages, e.g. at voltages up to 500 volts. One problem arising in this respect is that the air flowing onto the heat-emitting elements carries moisture and/or dirt, which may enter the heating device and cause there an electric flashover, i.e. a short circuit. Another fundamental problem is that persons who work in the area of the heating device have to be protected against the current-carrying parts of the heating device and of the heat-generating element, respectively.
WO 99/18756 discloses an immersion heater with PTC heating elements which are arranged between electric conductors and covered with insulating layers so as to insulate said electric conductors with respect to the metal housing of the immersion heater. In the case of this prior art, the housing sealingly encloses the PTC heating elements. For the purpose of insulation, a plate made of an insulating ceramic material is provided between the housing and the respective heat-generating element.